EARC: Enhanced Adaptation of Link Rate and Contention Window for IEEE 802.11 Multi-Rate Wireless Networks

IEEE 802.11 wireless network supports multiple link rates at the physical layer. Each link rate is associated with a certain required Signal-to-Interference-and-Noise Ratio (SINR) threshold for successfully decoding received packets. On transmission failures, the 802.11 DCF performs a binary exponential backoff mechanism to discourage channel access attempts, hoping to reduce congestion. When traditional link adaptation is applied, both rate reduction and binary backoff represent double penalties for this wireless link, which may cause overly conservative transmission attempts. On the other hand, once transmission succeeds, 802.11 DCF resets the backoff contention window to the minimum value to encourage channel access attempts. At the same time, traditional link adaptation may also decide to increase the data rate, which leads to overly aggressive transmission attempts. We observe this improper interaction of link rate and backoff mechanism that harms the 802.11 system performance, due to separate consideration of those two parameters. In this paper, we propose to jointly adapt the rate and backoff parameters. Specifically, an Enhanced Adaptation of link Rate and Contention window, abbreviated as EARC, is devised. EARC is a closed-loop (receiver-assisted) link rate adaptation protocol that jointly considers the backoff mechanism. With only one extra byte carried by the DATA packet, EARC incurs little controlling overhead despite its receiver-assisted nature. Moreover, since SINR information commonly utilized by receiver-assisted protocols is not precisely supported in real devices, we introduce a rate selection reference (RSR) table empirically derived by constantly monitoring the environmental energy level and reception behavior. The RSR table then guides the receiver to select the best sustainable rate for the transmitter. Simulation results demonstrate the RSR table is a practical option for making the rate decision, and the proposed EARC approach is effective in maintaining high system throughput, compared to other link adaptation algorithms.